1. Field of the Invention
The present invention relates to an image pickup device and drive method thereof.
2. Related Background Art
Research and development of an image sensor called a CMOS type image sensor which reads out a photoelectric conversion signal not with a CCD (charge coupled device) but with a MOS transistor, are becoming active nowadays.
This CMOS type image sensor is expected as an image sensor particularly for portable application because a CMOS logic LSI process can be used to fabricate the image sensor of such the type, a peripheral circuit can be integrated on a chip, and the CMOS type image sensor can be driven at a low voltage with a low power consumption. If a CMOS type image sensor is applied to technical fields requesting a high image quality, such as digital camera image pickup elements, it is necessary to provide some countermeasures against fixed pattern noises and random noises in order to obtain an image having a high S/N ratio.
As such countermeasures, a noise eliminating method has been proposed as disclosed, for example, in IEEE Journal of Solid-State Circuits, Vol. 32, No. 2, 1997 (drawings of FIG. 6 of this paper are shown in FIGS. 1A and 1B). In a circuit for reading out photoelectric conversion charges shown in FIG. 1A, light incident upon a photo-gate PG of a photoelectric conversion unit generates electric charges which are transferred from the photo-gate PG to an amplifier unit MIN via a transfer unit TX. The selection unit MX and a load current source MLN are activated to operate the amplifier unit MIN as a source follower. The charges amplified by the amplifier unit MIN are accumulated in capacitors CS and CR as photoelectric conversion charges and noise components immediately after resetting, respectively, and output as VOUTS and VOUTR.
The gate of the amplifier unit MIN has a floating diffusion FD gate region whose potential is reset by a reset unit MR to VDD when necessary. Switching transistors MSHS and MSHR are provided for transferring photoelectric conversion charges and noise components to the capacitors CS and CR respectively, during the operation of the source follower. Amplifier units MP1, MY1 and MLP1 output the amplified signal components VOUTS, and amplifier units MP2, MY2 and MLP2 output the amplified noise components VOUTR. The noise components VOUTR are thereafter subtracted from the signal components VOUTS to obtain the signal components from which the noise components are removed.
In this circuit constructed as above, as shown in FIG. 1B, after an input to the amplifier unit MIN is reset (after a gate signal R is changed to a low level), a gate signal SHR is applied to the gate of the switching transistor MSHR to turn on the switching transistor and read the noise components into the capacitor CR (during a period of a high level SHR). Thereafter, a low level signal is applied to the photo-gate PG which is a switch for controlling to transfer signal charges from the photodiode to the floating diffusion layer FD, and thereby charges remaining after the signal charges are read out to the FD are stored in the capacitor CS via the switching transistor MSHS. A difference (VOUTS−VOUTR) between two signals is finally output as the signal output.
A CMOS type image sensor is basically manufactured by CMOS logic processes and driven at a lower voltage than that of CCD. It is therefore difficult to form a sufficient potential gradient between the photodiode and charge accumulation region when the signal charges are transferred. Pixels unable to realize a sufficient transfer operation may be manufactured in some cases because of variation in manufacture processes. In such cases, fixed pattern noises (FPN) are generated because of variation in residual image characteristics of solid state image pickup elements. Irregularity of light output response characteristics is therefore a problem to be solved.